1. Field of the Invention
The present invention relates to a magnetic resonance imaging method and apparatus, and more particularly to a magnetic resonance imaging method and apparatus suitable for fast imaging of an object.
2. Description of the Prior Art
Magnetic resonance imaging technology is applied to the obtaining of sectional images of an object by utilizing a nuclear magnetic resonance phenomenon and is well known as what is useful for examining the human body.
A fast imaging method called "RARE" is disclosed in Magnetic Resonance in Medicine 3, 823-833 (1986). According to the fast imaging method, a number of echoes resulting from repeatedly applying a 180.degree. radio frequency pulse to the object after it is excited with a 90.degree. radio frequency pulse are differently phase-encoded, whereby one image is reconstructed. Typically, 128 or 256 echoes are employed to reconstruct one image. Accordingly, the time required for imaging is reduced to 1/n (n:the number of echoes to be used) as compared with what is required in any conventional spin echo method in which the excitation is needed 128 or 256 times to obtain 128 or 256 projections necessary for one image to be reconstructed. Since the magnitude of echoes is exponentially decreased in accordance with transverse relaxation time (T2) characteristics, however, the image obtained through this method tends to become significantly different in quality from what is obtained through the ordinary spin echo method.
A so-called multishot RARE method has been proposed to make such a problem less serious. With this method, the number of echoes is set to be smaller than that of projections. According to this method, however, artifacts are caused in a phase encoding direction by motion or flow of spins, resulting in the degradation of image quality of the object.